Uncovering the Brain's Reward System

Columbia’s neuroscience discovery breaks conventional wisdom.

Understanding how the brain’s reward system works helps to advance scientific research in neurodegenerative diseases such as Parkinson’s and the fields of medicine, biotechnology, pharmaceuticals, and psychiatry overall. Today, neuroscientists announced a new discovery that alters commonly held views on the brain’s reward learning system. Reward learning is not confined to select brain regions. Instead, the brain initiates learning complex associations at the somatosensory cortex, the area that handles the sense of touch. The neuroscience research team from Columbia University's Mortimer B. Zuckerman Mind Brain Behavior Institute consisting of Randy Bruno, Clay Lacefield, Eftychios Pnevmatikakis, and Liam Paninski published their findings on Cell Reports.

When you feel the sensations of touch, it’s due to the somatosensory cortex, a set of modules of the brain’s neocortex. Conventional neuroscientific views are that the sensory cortex plays a relatively straightforward role in the brain—that it mainly serves to transmit sensory information to the association cortex, which then goes to the frontal cortex for advanced processing. The sensory cortex, until now, has not been associated with the brain’s reward learning system.

The researchers studied mice with artificially introduced DNA that enables fluorescent indicators used to image neuronal activity. Given the barriers and challenges to conducting neuroscientific research on humans, rodents are often used in studies as a mammalian proxy. In the study, the dendrites (branch-like extensions of neurons) in the somatosensory cerebral cortex of mice were monitored.

The neuroscientists first trained the mice to locate a pole in a darkened room (sensory task), then move a lever to release water (the reward). As expected, the dendrites in the somatosensory cortex were excited when the mice’s whiskers came in contact with the pole. What was surprising and unexpected was that the same dendrites fired again when the mice received the reward of a water droplet.

The team then removed the pole and gave the trained mice water rewards. Even more unanticipated was that the trained mice’s somatosensory dendrites lit up when given the water reward even in the absence of a pole. In mice that were not trained in the whisker-task, the water reward did not fire the somatosensory dendrites. These scientific observations led the team to conclude that the association was acquired, or learned.

Interestingly, this breakthrough may lead to the discovery of a new neurotransmitter one day. Chemicals that influence the brain reward pathway play a major role in motivation and drive in response to rewarding stimuli. Neurotransmitters are chemical messengers synthesized and released by neurons to send signals to other neurons, glands or muscles. Examples of neurotransmitters include serotonin, histamine, oxytocin, endorphins, epinephrine (adrenaline), and dopamine.

According to the paper’s senior author Randy Bruno, Ph.D., there isn’t much dopamine in the somatosensory cortex. Dr. Bruno reasons that a yet-to-be-discovered neuromodulator in the somatosensory cortex functions in a similar manner to dopamine in other areas of the brain’s reward learning system. He plans to focus his research in this area next.

Some of the most addictive substances, such as nicotine, alcohol, heroin, cocaine, and barbiturates, activate the brain’s dopamine system. The discovery of a new neuromodulator that functions like dopamine in the brain’s reward learning system may help find novel ways to address substance abuse and addiction in the future.